Weather: Types of Precipitation

Types of Precipitation

Precipitation can take on many different forms. The temperature plays a major role in determining the various types of precipitation. Also important are the motions within the atmosphere. Raindrops aren't the only elements that could be falling upon our heads.

Snow

Precipitation can take many forms. Ice crystals can link to each other and form snowflakes. John Burroughs once said, "I was born with a chronic anxiety about the weather." Most people share that anxiety, and—once you're out of school and the possibility of "snow days"—few elements of weather bring on as much anxiety as snow. Although snow is beautiful, it is disruptive to traffic and communities. My own students who work part-time at grocery stores say they can tell how much snow I'm predicting based on the length of the check-out lines. The perceived need for loaves of bread and quarts of milk is directly proportional to the amount of snow in the forecast.

Snow has been a focus of study for centuries. In the sixteenth century, Swedish historian Olaus Magnus was the first to study individual snow crystals. Later French philosopher/mathematician Ren Descartes and even German astronomer Johannes Kepler studied the shapes and sizes of snowflakes. But not until the turn of the twentieth century did William Bentley put together a detailed catalogue of snowflake forms. Bentley was an American farmer who was completely fascinated by snowflakes. From age 20, he dedicated 40 years of his life to building a collection of 6,000 photographs of snow crystals—all different. An infinite variety of crystal shapes exists. Two snowflakes can look alike, but there is no shortage of different forms. Temperature and humidity determine the shape of snowflakes, along with the nature of the nuclei around which the crystals form.

Sleet and Hail

These two elements are confused more than any other precipitation types. Both are pieces of ice, but there are major differences between the two. If rain freezes into pellets while falling to the ground, those pellets are called sleet. Sleet occurs in the winter when warm air streams over a cold layer. Rain falls from the warm layer, then the drops pass through a shallow freezing layer that isn't exceptionally cold, so snowflakes are unable to form—just those pellets. During many winter storms, the precipitation will begin as snow, but as the warm air reaches overhead, the snow changes to sleet. As the warmer air becomes even more dominant, the sleet changes to rain. Just before the change from snow to sleet, the snowflakes become thick and heavy.

Sleet is quite different from hail, which is a warm-weather phenomenon. Hail forms when ice slowly descends through a cumulonimbus cloud. Strong updrafts allow layer after layer of ice to build up until the hail becomes so heavy that it overcomes the updrafts and falls to the ground. The hail can be tossed around the clouds as well, between melting and freezing layers, so layers of clear and opaque ice form—something like an onion skin.

Freezing Rain

When rain freezes on contact with surfaces that are below freezing, it's called "freezing rain." The air may be warm aloft, too warm for snow or sleet. But because of stubborn cold air at ground level, the rain forms a sheet of ice on the ground. A glaze of ice coats roads and walkways. When the ice build-up becomes extensive, widespread power outages occur. The weight of the ice can cause tree limbs to break as well as power lines.